As is well known, fabricating structures on land is less expensive and risky than doing the same work offshore. However, during the construction of offshore structures, a certain amount of offshore assembly time is generally required. To minimize these costs, these structures are often subdivided into a few large components.
One such component of an offshore structure is the substructure, which generally extends from the sea floor to an elevation above the water surface. The substructure is normally fabricated on its side onshore, skidded onto a barge, towed to the site, launched from the barge, upended to a vertical orientation while floating, and set on the bottom by ballasting. Piling is then driven into the sea floor through features in the substructure afterwhich the substructure is secured to the driven piling by grouting, welding, or other mechanical means.
In most cases, the substructure is a cantilever tower, but there are certain circumstances, however, when a braced tower is a more efficient (lighter) structure. In order to realize savings from the lighter weight of the braced tower, a low risk, economical method of fabrication and installation must be devised.
Methods already have been developed which involve assembling braced towers at the offshore site. Under these methods, the central tower and the braces, which are each independently fabricated onshore, are separately transported to the offshore site. After launching, each brace is connected to the tower thereby completing the substructure. Assembling the substructure in this fashion is both expensive and risky because of the length of time required and because a storm can strike at any time.
It is thus an object of this invention to devise a braced tower that can be fabricated entirely onshore in a compact bundle, transported to the offshore installation site, placed in the water, and opened to its final configuration while floating. Another object of this invention is to devise a braced substructure that can be deployed and installed with an absolute minimum of offshore assembly and/or construction time required. A further object of this invention is to provide a compact substructure that can be towed to the site and launched without requiring elaborate reconfiguration of the transport vessel. A further object of this invention is to provide sufficient clearances such that piling can be driven to anchor this unfolded structure in place. These and other advantages will become apparent upon further investigation.